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2 RECTANGULAR WAVE GUIDE ATTENUATORS TUNABLE PROBE
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Optical & Microwave Lab Manual

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Page 1: Optical & Microwave Lab Manual

2

RECTANGULAR WAVE GUIDE

ATTENUATORS

TUNABLE PROBE

Page 2: Optical & Microwave Lab Manual

3

STUDY OF MICROWAVE COMPONENTS

AIM:

To study the various microwave components structure and its application.

RECTANGULAR WAVE GUIDE

Wave guides are manufactured to the highest mechanical and electrical

standards and mechanical tolerances. L and S band wave guides are fabricated

by precision brazing of brass-plates and all other wave guides are in extrusion

quality.

ATTENUATORS

Attenuators are passive resistive elements that do the opposite of

amplifiers, they kill gain. Why would you want to do that? Suppose your design

specification calls for 10 dB gain, with a 1.2:1 maximum VSWR. You search

the amplifier vendors, and locate an amplifier in your frequency band, but it has

14.5 dB gain and a lousy 2.5:1 match on the input. By adding an attenuator to

the input, you can bring the gain down to 10 dB, and you will be improving the

input match. Two things to consider when you play this game: don‟t add an

attenuator to an amplifier‟s input if you are concerned with the amplifier‟s noise

figure, every dB of attenuation you put on the input will raise the noise figure

by the same amount. Similarly, don‟t add an attenuator to a power amplifier‟s

output without considering what it will do to your output power, or what the RF

output power of the power amp might do to your attenuator.

TUNABLE PROBE

Tunable probe is designed for use with model 6051 slotted sections.

These are meant for exploring the energy of the EF in a suitably fabricated

section of wave guide. The depth of penetration into a wave guide - section is

adjustable by the knob of the probe. The tip pick up the RF power from the line

and this power is rectified by crystal detector, which is then fed to the VSWR

meter or indicating instrument.

Page 3: Optical & Microwave Lab Manual

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DETECTOR MOUNT

ISOLATOR

CIRCULATOR

Page 4: Optical & Microwave Lab Manual

5

WAVE GUIDE DETECTOR MOUNT (TUNABLE)

Tunable Detector Mount is simple and easy to use instrument for detecting

microwave power through a suitable detector. It consists of a detector crystal mounted

in a section of a Wave guide and shorting plunger for matching purpose. The output

from the crystal may be fed to an indicating instrument. In K and bands detector

mounts the plunger is driven by a micrometer.

ISOLATOR

An isolator is a nonreciprocal transmission device that is used to isolate one

component from reflections of other components in the transmission line. An ideal

isolator completely absorbs the power for propagation in one direction and provides

lossless transmission in the opposite direction. Thus the isolator is usually called

uniline. Isolators are generally used to improve the frequency stability of microwave

generators such as klystrons and magnetrons in which the reflection from the load

affects the generating frequency. In such cases the isolator is placed between the

generator and load to prevent the reflected power from the unmatched load from

returning to the generator. As a result the isolator maintains the frequency stability of

the generator.

CIRCULATOR

A circulator is a ferrite device (ferrite is a class of materials with strange

magnetic properties) with usually three ports. The beautiful thing about circulators is

that they are non-reciprocal. That is, energy into port 1 predominantly exits port 2,

energy into port 2 exits port 3, and energy into port 3 exits port 1. In a reciprocal

device the same fraction of energy that flows from port 1 to port 2 would occur to

energy flowing the opposite direction, from port 2 to port 1. The selection of ports is

arbitrary, and circulators can be made to "circulate" either clockwise (CW) or

counterclockwise (CCW).

SLIDE SCREW TUNERS

Slide screw tuners are used for matching purposes by changing the penetration

and position of a screw in the slot provided in the centre of the wave guide. These

consists of a section of wave guide flanged on both ends and a thin slot is provided in

the broad wall of the Wave guide. A carriage carrying the screw, is provided over the

slot. A VSWR upto 20 can be tuned to a value less than 1.02 at certain frequency.

Page 5: Optical & Microwave Lab Manual

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DIRECTIONAL COUPLERS

MULTIHOLE DIRECTIONAL COUPLERS

MAGIC TEE

Page 6: Optical & Microwave Lab Manual

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DIRECTIONAL COUPLERS

A directional coupler is a four-port waveguide junction. It consists of a

primary waveguide and a secondary waveguide. When all the ports are

terminated in their characteristic impedance there is no free transmission of

power, without reflection between port 1 and port 2, and there is no

transmission of power between ports 1 and port 3 and between ports 2 and 4

because no coupling exists between these two pair of ports. The degree of

coupling between ports 1 and 4 and between ports 2 and 3 depends on the

structure of the coupler.

MULTIHOLE DIRECTIONAL COUPLERS

Multihole directional couplers are useful for sampling a part of

Microwave energy for monitoring purposes and for measuring reflections and

impedance. These consists of a section of Wave guide with addition of a second

parallel section of wave guide thus making it a four port network. However the

fourth port is terminated with a matched load. These two parallel sections are

coupled to each other through many holes almost to give uniform coupling;

minimum frequency sensitivity and high directivity.

Magic Tees (Hybrid tees)

A magic tee is a combination of the E-plane tee and H-plane tee. The

magic tee has several characteristics.

1. If the two ports of equal magnitude and the same phase are fed into port 1 and

port 2, the output will be zero at port 3 and additive at port 4.

2. If a wave is fed into port 4 (H arm), it will be divided equally between port 1

and port 2 of the collinear arms and will not appear in port 3.

3. If a wave is fed into port 3 (E arm), it will produce an output of equal

magnitude and opposite phase at port 1 and port 2. The output at port 4 is zero.

4. If a wave is fed into one of the collinear arms at port 1 or port 2, it will not

appear in the other collinear arm at port 2 or port 1 because the E- arm causes a

phase delay while the H- arm causes a phase advance.

Page 7: Optical & Microwave Lab Manual

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H-Plane Tee

E-Plane Tee

MOVABLE SHORT

Page 8: Optical & Microwave Lab Manual

9

H-plane tee (shunt tee)

An H-plane tee is a waveguide tee in which the axis of the side arm is

shunting the E- field or parallel to the H field of the main guide. It can be seen

that if the two input waves are fed in port 1 and port 2 of the collinear arm, the

output wave at port 3 will be in phase and additive. On the other hand, if the

input is fed into port 3, the wave will split equally into port 1 and port 2 in phase

and in same magnitude.

E-plane tee (series tee)

An E-plane tee is a waveguide in which the axis of the side arm is parallel

to the E- field of the main guide. If the collinear arms are symmetric about the

side arm, there are two different transmission characteristics.

MOVABLE SHORT

movable shorts consists of a section of waveguide, flanged on one end

and terminated with a movable shorting plunger on the other end. By means of

this non contacting type plunger, a reflection co-efficient of almost unity may be

obtained.

WAVEGUIDE BENDS

The size, shape, and dielectric material of a waveguide must be constant

throughout its length for energy to move from one end to the other without

reflections. Any abrupt change in its size or shape can cause reflections and a

loss in overall efficiency. When such a change is necessary, the bends, twists,

and joints of the waveguides must meet certain conditions to prevent reflections.

Page 9: Optical & Microwave Lab Manual

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MATCHED TERMINATION

HORN ANTENNA

KLYSTRON TUBE

Page 10: Optical & Microwave Lab Manual

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MATCHED TERMINATION

It consists of a small and highly dissipative taper flap mounted inside the centre

of a section of wave guide. Matched Terminations are useful for USWR measurement

of various waveguide components. These are also employed as dummy and as a

precise reference loads with Tee junctions, directional couplers and other similar

dividing devices.

PYRAMIDAL WAVEGUIDE HORN ANTENNA

Pyramidal Wave guide Horn antenna consists of waveguide joined to pyramidal

section fabricated from brass sheet. The pyramidal section shapes the energy to

concentrate in a specified beam. Wave guide horns are used as feed horns as radiators for

reflectors and lenses and as a pickup antenna for receiving microwave power.

KLYSTRON

A klystron is a specialized vacuum tube (evacuated electron tube) called a

linear-beam tube. The pseudo-Greek word klystron comes from the stem form klys of

a Greek verb referring to the action of waves breaking against a shore, and the end of

the word electron.

Frequency Meter

The cylindrical cavity forms a resonator that produces a suck-out in the frequency

response of the unit. This you would turn the knob until a dip in the response is observed.

The graduations will tell you what frequency you are at. Waveguide frequency meters use

a short circuit resonant cavity, which resonates at half wavelength. Most wave meters are

waveguide, however, coaxial types are possible. Waveguide wave meters can only

measure frequency over their respective frequency band.

Gunn Diode

A Gunn diode, also known as a transferred electron device (TED) is a form of

diode used in high-frequency electronics. It is somewhat unusual in that it consists only of

N doped semiconductor material, whereas most diodes consist of both P and N-doped

regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each

terminal, with a thin layer of lightly doped material in between. When a voltage is applied

to the device, the electrical gradient will be largest across the thin middle layer.

Eventually, this layer starts to conduct, reducing the gradient across it, preventing further

conduction. In practice, this means a Gunn diode has a region of negative differential

resistance.

Page 11: Optical & Microwave Lab Manual

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Page 12: Optical & Microwave Lab Manual

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VSWR meter

The SWR meter or VSWR meter measures the standing wave ratio in a

transmission line. This is an item of radio equipment used to check the quality

of the match between the antenna and the transmission line. The VSWR meter

should be connected in the line as close as possible to the antenna. This is

because all practical transmission lines have a certain amount of loss, causing

the reflected power to be attenuated as it travels back along the cable, and

producing an artificially low VSWR reading on the meter. If the meter is

installed close to the antenna, then this problem is minimized.

RESULT:

Page 13: Optical & Microwave Lab Manual

14

Block Diagram:

Klystron Power supply

Klystron

Mount

Isolator Frequency

meter

Variable

attenuator

Detector

Mount

Multi meter

VSWR

meter

CRO

Klystron Tube

Page 14: Optical & Microwave Lab Manual

15

REFLEX KLYSTRON CHARACTERISTICS

AIM:

To study the mode characteristics of the reflex klystron tube and to

determine its Electronic tuning range.

EQUIPMENT REQUIRED:

1. Klystron power supply

2. Klystron tube 2k-25 with klystron mount

3. Isolator

4. Frequency meter

5. Detector mount

6. Variable Attenuator

7. Wave guide stand

8. VSWR meter

9. Oscilloscope

10. BNC Cable

THEORY:

The reflex klystron is a single cavity variable frequency microwave

generator of low power and low efficiency. This is most widely used in

applications where variable frequency is desired as

1. In radar receivers

2. Local oscillator in μw receivers

3. Signal source in micro wave generator of variable frequency

4. Portable micro wave links.

5. Pump oscillator in parametric amplifier

Voltage Characteristics: Oscillations can be obtained only for specific

combinations of anode and repeller voltages that give favorable transit time.

Power Output Characteristics: The mode curves and frequency

characteristics. The frequency of resonance of the cavity decides the frequency

of oscillation. A variation in repeller voltages slightly changes the frequency.

Page 15: Optical & Microwave Lab Manual

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OBSERVATION TABLE:

Beam Voltage :…………V (Constant)

Beam Current :………….mA

Repeller

Voltage (V)

Voltage,

Vrms

(mV)

Power,

P=Vrms2(10

3/R).

(mW)

Dip Frequency

(GHz)

EXPECTED GRAPH:

Page 16: Optical & Microwave Lab Manual

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EXPERIMENTAL PROCEDURE:

A. CARRIER WAVE OPERATION:

1. Connect the equipments and components as shown in the block diiagram.

2. Set the variable attenuator at maximum Position.

3. Set the MOD switch of Klystron Power Supply at CW position, beam voltage control

knob to fully anti clock wise and repeller voltage control knob to fully clock wise and

meter switch to „OFF‟ position.

4. Rotate the Knob of frequency meter at one side fully.

5. Connect the DC microampere meter at detector.

6. Switch “ON” the Klystron power supply, CRO and cooling fan for the Klystron tube.

7. Put the meter switch to beam voltage position and rotate the beam voltage knob

clockwise slowly up to 300 Volts and observe the beam current on the meter by

changing meter switch to beam current position. The beam current should not increase

more than 30 mA.

8. Change the repeller voltage slowly and watch the current meter, set the maximum

voltage on CRO.

9. Tune the plunger of klystron mount for the maximum output.

10. Rotate the knob of frequency meter slowly and stop at that position, where there is

less output current on multimeter. Read directly the frequency meter between two

horizontal line and vertical marker. If micrometer type frequency meter is used read

the micrometer reading and find the frequency from its frequency calibration chart.

11. Change the repeller voltage and read the current and frequency for each repeller

voltage.

B. SQUARE WAVE OPERATION:

1. Connect the equipments and components as shown in the block diagram.

2. Set Micrometer of variable attenuator around some Position.

3. Set the range switch of VSWR meter at 40 dB position, input selector switch to

crystal impedance position, meter switch to narrow position.

4. Set Mod-selector switch to AM-MOD position .beam voltage control knob to fully

anti clockwise position.

5. Switch “ON” the klystron power Supply, VSWR meter, CRO and cooling fan.

Page 17: Optical & Microwave Lab Manual

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Out Put:

Page 18: Optical & Microwave Lab Manual

19

6. Switch “ON” the beam voltage. Switch and rotate the beam voltage knob clockwise

up to 300V in meter.

7. Keep the AM – MOD amplitude knob and AM – FREQ knob at the mid position.

8. Rotate the reflector voltage knob to get deflection in VSWR meter or square wave on

CRO.

9. Rotate the AM – MOD amplitude knob to get the maximum output in VSWR meter or

CRO.

10. Maximize the deflection with frequency knob to get the maximum output in VSWR

meter or CRO.

11. If necessary, change the range switch of VSWR meter 30dB to 50dB if the deflection

in VSWR meter is out of scale or less than normal scale respectively. Further the

output can be also reduced by variable attenuator for setting the output for any

particular position.

C. MODE STUDY ON OSCILLOSCOPE:

1. Set up the components and equipments as shown in the block diagram.

2. Keep position of variable attenuator at min attenuation position.

3. Set mode selector switch to FM-MOD position FM amplitude and FM frequency

knob at mid position keep beam voltage knob to fully anti clock wise and reflector

voltage knob to fully clockwise position and beam switch to „OFF‟ position.

4. Keep the time/division scale of oscilloscope around 100 HZ frequency measurement

and volt/div. to lower scale.

5. Switch „ON‟ the klystron power supply and oscilloscope.

6. Change the meter switch of klystron power supply to Beam voltage position and set

beam voltage to 300V by beam voltage control knob.

7. Keep amplitude knob of FM modulator to max. Position and rotate the reflector

voltage anti clock wise to get the modes as shown in figure on the oscilloscope. The

horizontal axis represents reflector voltage axis and vertical represents o/p power.

8. By changing the reflector voltage and amplitude of FM modulation in any mode of

klystron tube can be seen on oscilloscope.

RESULT:

Page 19: Optical & Microwave Lab Manual

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BLOCK DIAGRAM

Gunn Power supply

Gunn Oscillator

PIN

Modulator

Variable attenuator

Detector

Mount

VSWR meter

Isolator

Page 20: Optical & Microwave Lab Manual

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GUNN DIODE CHARACTERISTICS

AIM:

To study the V-I characteristics of Gunn diode.

EQUIPMENT REQUIRED:

1. Gunn power supply

2. Gunn oscillator

3. PIN Modulator

4. Isolator

5. Frequency Meter

6. Variable attenuator

7. Slotted line

8. Detector mount and CRO.

THEORY:

Gunn diode oscillator normally consist of a resonant cavity, an

arrangement for coupling diode to the cavity a circuit for biasing the diode and a

mechanism to couple the RF power from cavity to external circuit load. A co-

axial cavity or a rectangular wave guide cavity is commonly used.

The circuit using co-axial cavity has the Gunn diode at one end at one end

of cavity along with the central conductor of the co-axial line. The O/P is taken

using a inductively or capacitively coupled probe. The length of the cavity

determines the frequency of oscillation. The location of the coupling loop or

probe within the resonator determines the load impedance presented to the Gunn

diode. Heat sink conducts away the heat due to power dissipation of the device.

EXPERIMENTAL PROCEDURE:

Voltage-Current Characteristics:

1. Set the components and equipments as shown in the block diagram.

2. Initially set the variable attenuator for minimum attenuation.

3. Keep the control knobs of Gunn power supply as below

Page 21: Optical & Microwave Lab Manual

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MODEL GRAPH:

OBSERVATION TABLE:

Gunn bias voltage

(V)

Gunn diode current

(mA)

Volts (V)

Threshold voltage

I-V CHARACTERISTICS OF GUNN OSCILLATOR

I (mA)

Page 22: Optical & Microwave Lab Manual

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Meter switch – “OFF”

Gunn bias knob – Fully anti clock wise

PIN bias knob – Fully anti clock wise

PIN mode frequency – any position

4. Set the micrometer of Gunn oscillator for required frequency of operation.

5. Switch “ON” the Gunn power supply.

6. Measure the Gunn diode current to corresponding to the various Gunn bias

voltage through the digital panel meter and meter switch. Do not exceed the

bias voltage above 10 volts.

7. Plot the voltage and current readings on the graph.

8. Measure the threshold voltage which corresponding to max current.

Note: Do not keep Gunn bias knob position at threshold position for more than

10-15 sec. readings should be obtained as fast as possible. Otherwise due to

excessive heating Gunn diode may burn

Page 23: Optical & Microwave Lab Manual

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Out Put:

Page 24: Optical & Microwave Lab Manual

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RESULT:

Page 25: Optical & Microwave Lab Manual

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Block Diagram:

Klystron Power supply

Klystron

Mount

Isolator Frequency

meter

Variable

attenuator

Slotted

Line

VSWR meter

Matched

Termination

Movable Short

Klystron Tube

Tunable Probe

Page 26: Optical & Microwave Lab Manual

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MEASUREMENT OF VSWR FREQUENCY AND

WAVELENGTH

AIM:

To determine the VSWR, frequency and wavelength in a rectangular wave

guide working in TE10 mode.

EQUIPMENT REQUIRED:

1. Klystron tube

2. Klystron power supply

3. Klystron mount

4. Isolator

5. Frequency meter

6. Variable attenuator

7. Slotted section

8. Tunable probe

9. VSWR meter

10. Wave guide stand

11. Movable Short

12. Matched termination

THEORY:

The cut-off frequency relationship shows that the physical size of the wave

guide will determine the propagation of the particular modes of specific orders

determined by values of m and n. The minimum cut-off frequency is obtained for a

rectangular wave guide having dimension a>b, for values of m=1, n=0, i.e. TE10 mode

is the dominant mode since for TMmn modes, n#0 or n#0 the lowest-order mode

possible is TE10, called the dominant mode in a rectangular wave guide for a>b.

For dominant TE10 mode rectangular wave guide λo, λg and λc are related as

below.

1/λo² = 1/λg² + 1/λc²

Where λo is free space wave length

λg is guide wave length

λc is cut off wave length

For TE10 mode λc – 2a where „a‟ is broad dimension of wave guide.

Page 27: Optical & Microwave Lab Manual

28

OBSERVATION TABLE:

B

eam

vo

ltag

e(v

)

Bea

m

curr

ent

(mA

)

Rep

elle

r

vo

ltag

e(v

)

fo (

usi

ng f

req

met

er)

(GH

Z)

d1

(cm)

d2

(cm)

d3

(cm)

d4

(cm)

∆d1=

d2-

d1

(cm)

∆d2=

d3-d2

(cm)

∆d3

=

d4 -

d 3

∆d

=(∆

d1

+∆

d2

+

∆d

3)/

3

λg

=2

x ∆

d

λo

(cm

)

fo (

HZ

)

Page 28: Optical & Microwave Lab Manual

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PROCEDURE:

1. Set up the components and equipments as shown in figure.

2. Set up variable attenuator at minimum attenuation position.

3. Keep the control knobs of klystron power supply as below:

Beam voltage – OFF

Mod-switch – AM

Beam voltage knob – Fully anti clock wise

Repeller voltage – Fully clock wise

AM – Amplitude knob – Around fully clock wise

AM – Frequency knob – Around mid position

4. Switch „ON‟ the klystron power supply, CRO and cooling fan switch.

5. Switch ‟ON‟ the beam voltage switch and set beam voltage at 300V with help of

beam voltage knob.

6. Adjust the repeller voltage to get the maximum amplitude in CRO

7. Maximize the amplitude with AM amplitude and frequency control knob of power

supply.

8. Tune the plunger of klystron mount for maximum Amplitude.

9. Tune the repeller voltage knob for maximum Amplitude.

10. Tune the frequency meter knob to get a „dip‟ on the CRO and note down the

frequency from frequency meter.

11. Replace the termination with movable short, and detune the frequency meter.

12. Move the probe along with slotted line. The amplitude in CRO will vary .Note and

record the probe position , Let it be d1.

13. Move the probe to next minimum position and record the probe position again, Let it

be d2.

14. Calculate the guide wave length as twice the distance between two successive

minimum position obtained as above.

15. Measure the wave guide inner board dimension „a‟ which will be around 22.86mm for

x-band.

16. Calculate the frequency by following equation.

22

11

cg

cf

Where C = 3x108 meter/sec. i.e. velocity of light.

Page 29: Optical & Microwave Lab Manual

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Page 30: Optical & Microwave Lab Manual

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17. Verify with frequency obtained by frequency modes

18. Above experiment can be verified at different frequencies.

fo = C/λo => C => 3x108 m/s (i.e., velocity of light)

1/λo² = 1/λg² + 1/λc²

220

cg

cg

λg = 2x ∆d

For TE10 mode => λc = 2a

a wave guide inner broad dimension

a = 2.286cm” (given in manual)

λc = 4.6cm

SWR is given by, SWR= λg/(π∆d)

RESULT:

Page 31: Optical & Microwave Lab Manual

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Block Diagram:

Klystron

Power supply

Klystron

Mount

Isolator Frequency

meter

Variable

attenuator

Klystron

Tube

Matched

Termination

Isolator/

Circulator

VSWR

Meter

Detector

Mount

Page 32: Optical & Microwave Lab Manual

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STUDY THE FUNCTION OF ISOLATOR

AIM:

To study the function of isolator by measuring the main line and auxiliary

line VSWR.

EQUIPMENT REQUIRED:

1. Microwave source Klystron tube

2. Isolator

3. Frequency meter

4. Variable attenuator

5. Slotted line

6. Tunable probe

7. Detector mount

8. Matched termination

9. Klystron power supply & Klystron mount

10. Cooling fan

11. BNC-BNC cable

13. VSWR or CRO

Theory:

Isolator: An isolator is a two-port device that transfers energy from input to

output with little attenuation and from output to input with very high

attenuation.

Circulator: The circulator is defined as a device with ports arranged such that

energy entering a port is coupled to an adjacent port but not coupled to other

ports. Refer to the fig. A wave incident on port 1 is coupled to port 2 only, a

wave incident at port 2 is coupled to port 3 only and so on.

Insertion loss

The ratio of power supplied by a source to the input port to the power detected

by a detector in the coupling arm, i.e. output arm with other port terminated in

the matched load, is defined as insertion loss or forward loss.

Page 33: Optical & Microwave Lab Manual

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Tabulation for Isolator

Line Power in dB

Input Line P1

Mail line P2

Auxiliary line P3

Page 34: Optical & Microwave Lab Manual

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Isolation

It is the ratio of power fed to input arm to the power detected at not coupled port with

other port terminated in the matched load.

Input VSWR

The input VSWR of an isolator or circulator is the ratio of voltage maximum to

voltage minimum of the standing wave existing on the line when one port of it

terminates the line and other have matched termination.

PROCEDURE:

1. Input VSWR Measurement

a. Set up the components and equipments as shown in the fig with input port of

isolator or circulator towards slotted line and matched load on other ports of it.

b. Energize the microwave .source for particular operation of frequency.

c. With the help of slotted line, probe and SWR meter. Find SWR, of the

isolator or circulator as described for low and medium SWR measurements.

d. The above procedure can be repeated for other ports or for other frequencies.

2. Measurement of Insertion Loss and Isolation

a. Remove the probe and isolator or circulator from slotted line and connect the

detector mount to the slotted section. The output of the detector mount should be

connected SWR meter.

b. Energize the microwave source for maximum output particular frequency of

operation. Tune the detector mount for maximum output in the SWR Meter.

c. Set any reference level of power in SWR meter with the help of variable

attenuator and gain control knob of SWR meter. Let it be P1.

d. Carefully remove the detector mount from slotted line without disturbing the

position of set up. Insert the isolator/circulator between slotted line and detector

mount. Keeping input port to slotted line and detector at its output port. A matched

termination should be placed a third port in case of circulator.

e. Record the reading in the SWR meter. If necessary change range -dB switch

to high or lower position. Let it be P2.

Page 35: Optical & Microwave Lab Manual

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Page 36: Optical & Microwave Lab Manual

37

f. For measurement of isolation, the isolator or circulator has to be

connected in reverse i.e. output port to slotted line and detector to input port with

another port terminated by matched termination (in case circulator) after setting a

reference level without isolator or circulator in the set up as described in insertion

loss measurement. Let it be P3.

Result and Analysis:

g. Compute insertion loss on P1 – P2 in dB.

h. Compute isolation as P1 - P3 in dB.

i. The same experiment can be done for other ports of circulator.

j. Repeat the above experiment for other frequencies if required

Result:

Page 37: Optical & Microwave Lab Manual

38

Block Diagram:

Klystron

Power supply

Klystron Mount

Isolator

Frequency meter

Variable

attenuator

Detector

Mount

CRO

Klystron Tube

Page 38: Optical & Microwave Lab Manual

39

ATTENUATION MEASUREMENT

AIM:

To study insertion loss and attenuation measurement of attenuator.

EQUIPMENT REQUIRED:

1. Microwave source Klystron tube

2. Isolator

3. Frequency meter

4. Variable attenuator

5. Slotted line

6. Tunable probe

7. Detector mount

8. Matched termination

9. Test attenuator

a) Fixed

b) Variable

10. Klystron power supply & Klystron mount

11. Cooling fan

12. BNC-BNC cable

13. VSWR or CRO

THEORY:

The attenuator is a two port bidirectional device which attenuates some power

when inserted into a transmission line.

Attenuation A (dB) = 10 log (P1/P2)

Where P1 = Power detected by the load without the attenuator in the line

P2 = Power detected by the load with the attenuator in the line.

PROCEDURE:

1. Connect the equipments as shown in the block diagram.

2. Energize the microwave source for maximum power at any frequency of operation

3. Connect the detector mount to the slotted line and tune the detector mount also for

max deflection on VSWR or on CRO.

Page 39: Optical & Microwave Lab Manual

40

MODEL GRAPH:

OBSERVATION TABLE:

Micrometer

reading

(mm)

Voltage,

Vrms1

(mV)

Power,

P1=Vrms2(10

3/R).

(mW)

Voltage,

Vrms2

(mV)

Power,

P2=Vrms2(10

3/R)

(mW)

Attenuation

= 10

log(P1/P2)

(dB)

Page 40: Optical & Microwave Lab Manual

41

4. Set any reference level on the VSWR meter or on CRO with the help of

variable attenuator. Let it be P1.

5. Carefully disconnect the detector mount from the slotted line without

disturbing any position on the setup place the test variable attenuator to the

slotted line and detector mount to O/P port of test variable attenuator. Keep

the micrometer reading of text variable attenuator to zero and record the

readings of VSWR meter or on CRO. Let it to be P2. Then the insertion

loss of test attenuator will be P1-P2 db.

6. For measurement of attenuation of fixed and variable attenuator. Place the

test attenuator to the slotted line and detector mount at the other port of test

attenuator. Record the reading of VSWR meter or on CRO. Let it be P3

then the attenuation value of variable attenuator for particular position of

micrometer reading of will be P1-P3 db.

7. In case the variable attenuator changes the micro meter reading and record

the VSWR meter or CRO reading. Find out attenuation value for different

position of micrometer reading and plot a graph.

8. Now change the operating frequency and all steps should be repeated for

finding frequency sensitivity of fixed and variable attenuator.

Note:1. For measuring frequency sensitivity of variable attenuator the position

of micrometer reading of the variable attenuator should be same for all

frequencies of operation.

RESULT:

Page 41: Optical & Microwave Lab Manual

42

Block diagram:

Klystron

Power supply

Klystron

Mount

Isolator Frequency

meter

Variable

attenuator

Matched

Termination

Klystron

Tube

Magic TEE Matched

Termination

Matched

Termination

Klystron

Power supply

Klystron

Mount

Isolator Frequency

meter

Variable

attenuator

Matched

Termination

Klystron

Tube

Magic TEE Detector

Mount

Matched

Termination

Page 42: Optical & Microwave Lab Manual

43

TO STUDY THE S - PARAMETER OF E-PLANE T, H-

PLANE T AND MAGIC T

AIM:

To determine isolations and coupling coefficients for E, H plane Tee and

Magic Tee junctions.

EQUIPMENT REQUIRED:

1. Microwave source Klystron tube

2. Isolator

3. Frequency meter

4. Variable attenuator

5. Magic Tee, E-Plane Tee and H-Plane Tee.

6. Tunable probe

7. Detector mount

8. Matched termination

9. Cooling fan

10. BNC-BNC cable

13. VSWR or CRO

THEORY:

H Plane Tee

An H-plane tee is a waveguide tee in which the axis of the side arm is

shunting the E- field or parallel to the H field of the main guide. It can be seen

that if the two input waves are fed in port 1 and port 2 of the collinear arm, the

output wave at port 3 will be in phase and additive. On the other hand, if the

input is fed into port 3, the wave will split equally into port 1 and port 2 in phase

and in same magnitude.

E Plane Tee

An E-plane tee is a waveguide in which the axis of the side arm is parallel

to the E- field of the main guide. If the collinear arms are symmetric about the

side arm, there are two different transmission characteristics.

Page 43: Optical & Microwave Lab Manual

44

Tabulation:

Pin=_________dB

Nature of

Tee Relative Power(dB) Isolation(Iij)dB

Coupling

Coefficient

Cij=10Iij/20

E-Plane

Tee

I-arm II-arm C12

III-arm C13

III -

arm

II -arm C32

I-arm C31

H-Plane

Tee

I-arm II-arm C12

III-arm C13

III -

arm

II -arm C32

I-arm C31

Pin=_____________dB

Magic Tee orientation

Pi(dB) Pj(dB) Iij(dB) Cij Input Arm-i

Output

Arm-j

1

2 I12 C12

3 I13 C13

4 I14 C14

2

1 I21 C21

3 I23 C23

4 I24 C24

3

4 I34 C34

1 I31 C31

2 I32 C32

4

3 I43 C43

1 I41 C41

2 I42 C42

Page 44: Optical & Microwave Lab Manual

45

Magic Tee

A magic tee is a combination of the E-plane tee and H-plane tee. The

magic tee has several characteristics.

1. If the two ports of equal magnitude and the same phase are fed into port 1 and

port 2, the output will be zero at port 3 and additive at port 4.

2. If a wave is fed into port 4 (H arm), it will be divided equally between port 1

and port 2 of the collinear arms and will not appear in port 3.

3. If a wave is fed into port 3 (E arm), it will produce an output of equal

magnitude and opposite phase at port 1 and port 2. The output at port 4 is zero.

4. If a wave is fed into one of the collinear arms at port 1 or port 2, it will not

appear in the other collinear arm at port 2 or port 1 because the E- arm causes a

phase delay while the H- arm causes a phase advance.

Input VSWR

Value of VSWR corresponding to each port, as load to the line while

other ports are terminated in matched load.

Isolation

The isolation between E and H arm is defined as the ratio of the power

supplied by the generator connected to the arm to the power detected at H arm

when side arm 1 and 2 are terminated in matched load.

Hence, isolation(dB)=10log10(P4/P3)

Similarly isolation between other parts may also be defined.

Coupling Coefficient

It is defined as Cij=10-α/20

Where, α=attenuation/isolation in dB,

iinput arm

joutput arm

Page 45: Optical & Microwave Lab Manual

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Page 46: Optical & Microwave Lab Manual

47

Procedure:

VSWR Measurement:

a. Set up the components and equipments as shown in fig. keeping E arm towards

slotted line and matched termination to other ports.

b. Energize the microwave source for particular frequency of operation and tune

the detector mount for maximum output.

c. Measure the SWR of E-arm as described in measurement of SWR for low and

medium value.

d. Connect another arm to slotted line and terminate the other port with matched

termination. Measure the SWR as above. Similarly, SWR of any port can be

measured.

Measurement of isolation and coupling coefficient:

a. Remove the tunable probe and Magic Tee from the slotted line and connect

the detector mount to slotted line.

b. Energize the microwave source for particular frequency of operation and

tune the detector mount for maximum output.

c. With the help of variable attenuator and gain control knob of SWR meter set

any power level in the SWR meter and note down. Let it be P3.

d. Without disturbing the position of variable attenuator and gain control knob,

carefully place the Magic Tee after slotted line keeping H-arm connected to

slotted line, detector to E arm and matched termination to arm 1 and 2. Note

down the reading of SWR meter. Let it be P4.

e. In the same way measure P1 & P2 by connecting detector on these ports one

by one.

f. Determine the isolation between port 3 and 4 as P3-P4 in dB.

g. Determine the coupling coefficient by P3- P1 for port P1 & P2.

h. Repeat the above experiment for other frequencies.

Result:

Page 47: Optical & Microwave Lab Manual

48

Block Diagram:

Klystron Power supply

Klystron Mount

Isolator Frequency

meter

Variable attenuator

Detector Mount

Klystron Tube

VSWR meter

Horn

Antennas

Page 48: Optical & Microwave Lab Manual

49

ANTENNA GAIN MEASUREMENT AND TO STUDY

THE RADIATION PATTERN OF AN ANTENNA

AIM:

To measure the polar pattern of a waveguide horn antenna.

Apparatus Required:

1. Microwave source Klystron tube

2. Isolator

3. Frequency meter

4. Variable attenuator

5. PIN Modulator.

6. Horn Antenna

7. Detector mount

8. Cooling fan

9. BNC-BNC cable

13. VSWR or CRO

THEORY:

If the transmission line propagating energy is left open at one end, there will

be radiation from this end. In case of a rectangular waveguide this antenna presents

a mismatch of about 2:1 and it radiates in many directions. The match will improve

if the open waveguide is a horn shape. The radiation pattern of an antenna is a

diagram of field strength or more often the power intensity as a function of the

aspect angle at a constant distance from the radiating antenna. An antenna pattern

is of course three dimensional pattern in one or several planes. An antenna pattern

consists of several lobes, the main lobe side lobes and back lobe as low as possible.

The power intensity at the maximum of the main lobe compared to the power

intensity achieved from an imagery omnidirectional antenna with the same power

fed to the antenna is defined as gain of the antenna.

3DB BEAMWIDTH

The angle between the two points on a main lobe were the power intensity is

half the maximum power intensity. When measuring antenna patterns it is normally

most interesting to plot the pattern far from the antenna.

Page 49: Optical & Microwave Lab Manual

50

Mode Graph:

Tabulation for radiation pattern:

Input Power, P1=______________dB

Angle

(left

side)

Relative

Power(dB)

Angle

(right

Side)

Relative

Power(dB)

P2 P2

Page 50: Optical & Microwave Lab Manual

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PROCEDURE

1. Arrange the apparatus as shown in the figure.

2. Ensure that all the knobs in the power supply and VSWR meter are of

minimum position

3. Switch on the Klystron power supply and wait for 2 minutes

4. Switch ON the beam switch and turn it to beam voltage position

5. Set the beam voltage at 250V

6. Switch ON the CRO and VSWR meter

7. Keep the switch SW3 at INT position

8. Adjust the repeller voltage knob (70V) so that a distorted waveform is

obtained

9. The operating repeller voltage is 70 V

10. Adjust the modulating frequency and the modulating amplitude to get a

perfect square wave

11. Find λg from slotted section

12. Replace the detector mount by horn, receiver horn was also placed on the

stand at some distance.

13. Turn the receiving horn to the left in 2o or 5o steps and take corresponding

VSWR dB reading.

14. Now turn the receiving side and repeat the above step 13

15. Draw a relative power pattern

16. From the diagram determine 3db width of the horn can be measured

17. Theoretical beam width was determined using the formula.

Gain Measurement:

1. Set up the equipments as shown in fig. Both horns should be in line. Connect

standard gain horn antenna (16dB) at transmitter end and any other antenna for

which gain is to be measured at the receiver end.

2. Keep the range dB switch of VSWR meter at appropriate position.

Page 51: Optical & Microwave Lab Manual

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Tabulation for Gain measurement:

Transmitted Power

(dB)

Received Power (dB)

Page 52: Optical & Microwave Lab Manual

53

3. Energize the Gunn Oscillator for maximum output at desired frequency with

modulating amplitude and frequency of potentiometer and by tuning of detector

4. Obtain maximum reading in SWR meter with variable attenuator. Record this

reading as Pr (received power).

5. Replace the transmitting horn by detector mount and change the appropriate

range db position to get the reading (do not touch the gain control knob) Note

and record the range db position and reading as Pt.

6. Now change the horn antenna at the receiver end.

RESULT:

Page 53: Optical & Microwave Lab Manual

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Block Diagram

Tabulation:

Sl.No Input LED Voltage(Volt) Output Power(dB)

A

Page 54: Optical & Microwave Lab Manual

55

DC CHARACTERISTICS OF LED AND PIN PHOTO

DIODE

Aim:

The objective of this experiment is to plot the V-I Characteristics of Photo LED

and characteristic of Photo Detector.

Apparatus Required:

1. Optical fiber trainer kit

2. Optical fiber

3. Voltmeter

4. Ammeter

Theory:

LED

LED‟s and LASER diodes are the commonly used sources in optical

communication systems, whether the system transmits digital or analog signal. It is

therefore, often necessary to use linear electrical to optical converter to allow its use in

intensity modulation & high quality analog transmission systems. LED's have a linear

optical output with relation to the forward current over a certain region of operation.

Photo Diode

Photo Transistors and Photo Diodes are the commonly used detectors in optical

communication systems, whether the system receives digital or analog signal. It is

therefore, often necessary to use linear optical to electrical converter to allow its use in

intensity demodulation & high quality analog receiving systems. Photo Diodes have a

linear electrical output with relation to the light intensity over a certain region of

operation.

Procedure for VI characteristics of LED:

1. Connect power supply to the board.

2. Ensure that all switched faults are in OFF condition.

3. Put emitter 1 block in Digital Mode

4. Make connections as shown in the Block Diagram.

a. Connect the bias 1 preset of comparator to the emitter 1 input.

b. Adjust the bias 1 preset to its minimum setting fully counter clockwise.

5. Now look down the emitter 1 LED Socket and slowly advance the setting of

the bias 1 preset until in subdued lighting the light from LED is just visible.

6. Connect the DMM between + 12V supply (Red Socket) and tp of Input of

Emitter LED. The DMM will now read the forward voltage (V f)

Page 55: Optical & Microwave Lab Manual

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Block Diagram:

Tabulation:

Sl.No Input LED

Voltage(Volt)

Photo detector

Current(mA)

Page 56: Optical & Microwave Lab Manual

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7. Measure the voltage drop across the 1k (R9) current limiting resistors by

connecting DMM between tp of Input of Emitter LED and (tp3). The forward

current is given by dividing the readings by 1k. This If is known as threshold

current.

8. Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), note

the corresponding If (forward current).

9. Record these values of Vf and If & plot the characteristic between these two.

Procedure for Photo Detector:

1. Connect power supply to the board.

2. Ensure that all switched faults are in OFF condition.

3. Put emitter 1 block in Digital Mode

4. Make connections as shown in the Block Diagram.

a. Connect the bias 1 preset of comparator to the emitter 1 input.

b. Adjust the bias 1 preset to its minimum setting fully counter clockwise.

8. Now look down the emitter 1 LED Socket and slowly advance the setting of

the bias 1 preset until in subdued lighting the light from LED is just visible.

c. Connect the fiber optic cable between emitter output and detectors input.

5. Connect the DMM between + 12V supply (Red Socket) and tp of Input of

Emitter LED. The DMM will now read the forward voltage (V f)

6. Measure the voltage drop across the 75E resistors by connecting DMM

between tp of output of Photo Transistor and Ground. The detector current is

given by dividing the readings by 75E.

7. Vary the bias 1 preset so as to vary the forward voltage (as 1.0, 1.5…4.0), not

the corresponding If (forward current).

8. Record these values of Vf and Id & plot the characteristic between these two.

Result:

Page 57: Optical & Microwave Lab Manual

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Block diagram: Study of Propagation Loss

Study of Bending Loss

Page 58: Optical & Microwave Lab Manual

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STUDY OF BENDING LOSS AND PROPAGATION

LOSS IN OPTICAL FIBRE

AIM:

The objective of this experiment is to measure the propagation loss and the

bending loss in the optical fiber.

Apparatus Required:

1. Optical fiber trainer kit

2. Optical fiber of different lengths

3. Power meter

4. CRO

Theory:

Attenuation loss (or path propagation loss) is the reduction in power density

(attenuation) of an electromagnetic wave as it propagates through space. Attenuation

loss is a major component in the analysis and design of the link budget of a

telecommunication system.

Attenuation occurring as a result of either a bend in an optical fibre that

exceeds the minimum bend radius or an abrupt discontinuity in the core/cladding

interface is called bending loss. The incident light rays strike the boundary between

the core and the cladding at an angle less than the critical angle and enter the cladding,

where they are lost

Procedure:

i)To find propagation loss:

1. Connect the power supply to the board.

2. Make the following connections

a) Function generators 1KHz sine wave output to input 1 socket of emitter 1 circuit

via 4mm lead.

b) Connect 0.5m optic fibre between emitter 1 output and detector 1‟s input.

c) Connect detector 1 output to amplifier 1 input socket via 4mm lead.

3. Switch ON the power supply.

4. Set the oscilloscope channel 1 to 0.5V /div and adjust 4-6 div amplitude by using

x1 probe with the help of variable pot in function generator block input 1 of emitter 1.

5. Observe the output signal from detector t p 10 on CRO.

Page 59: Optical & Microwave Lab Manual

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Tabulation:

Attenuation Losses:

Input Amplitude

V1(Volt)

Output Amplitude

V2(Volt)

Fiber-1_____Length,

L1(metre)

Fiber-2_____Length,

L2(metre)

Bending Losses:

Radius of curvature,

R(metre) Input Amplitude

V1(Volt)

Output Amplitude

V2(Volt)

Page 60: Optical & Microwave Lab Manual

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6. Adjust the amplitude of the received signal as that of transmitted one with the

help of gain adjust

pot in AC amplifier block. Note this amplitude and name it V1.

7. Now replace the previous FO cable with 1m cable without disturbing any

previous setting.

8. Measure the amplitude at the receiver side again at output of amplifier 1

socket t p 28. Note this

value and name it V2.

9. Calculate propagation (attenuation) loss with the help of following formula

V1/V2 = exp(-α(L1+L2))

Where α is loss in nepers/m

1 neper = 8.686dB

L1 = length of shorter cable (0.5m)

L2 = length of longer cable (1m)

ii)To find bending loss

1. Repeat all steps from 1-6 of the above procedure using 1m cable.

2. Wind FO cable on the Mandrel and observe the corresponding AC

amplifier output on CRO.

It will be gradually reducing showing loss due to bends.

RESULTS:

Page 61: Optical & Microwave Lab Manual

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SETTING UP FIBER OPTIC ANALOG LINK

Emitter circuit Detector circuit

Function Generator AC Amplifier

1 KHz Circuit

OBSERVATION

Input Voltage

(V)

Output Voltage

(V)

Time

(ms)

Out

Page 62: Optical & Microwave Lab Manual

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FIBER OPTIC ANALOG AND DIGITAL LINK

SETTING UP FIBER OPTIC ANALOG LINK

AIM:

The objective of this experiment is to study a 650 nm fiber optic analog

link. In this experiment, we will study a relationship between the input signal

and the received signal.

Apparatus Required:

1. Optical fiber trainer kit

2. Optical fiber

3. CRO

PROCEDURE:

1. Connect the power supply to the board.

2. Ensure that all switch faults are OFF.

3. Make the following connections.

a. Connect the function generator 1 KHz sine wave output to the

emitter 1‟s input.

b. Connect the fiber optic cable between the emitter‟s output and

detector‟s input.

c. Connect detector‟s output to the AC amplifier 1‟s input.

4. On the board switch emitter 1‟s driver to analog mode.

5. Switch ON the power.

6. Observe the input to emitter (TP 5) with output from AC amplifier 1

(TP19) and note that the two signals are same.

RESULT:

Page 63: Optical & Microwave Lab Manual

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SETTING UP FIBER OPTIC DIGITAL LINK

Emitter circuit Detector circuit

AC amplifier

Circuit

OBSERVATION

Input Voltage

(V)

Output Voltage

(V)

Time

(ms)

Function Generator 1 KHz

Out

Comparator

Page 64: Optical & Microwave Lab Manual

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SETTING UP FIBER OPTIC DIGITAL LINK

AIM:

The objective of this experiment is to study a 650 nm fiber optic digital link. In

this experiment, we will study a relationship between the input signal and the received

signal.

Apparatus Required:

1. Optical fiber trainer kit

2. Optical fiber

3. CRO

PROCEDURE:

1. Connect the power supply to the board.

2. Ensure that all switch faults are OFF.

3. Make the following connections.

a. Connect the function generator 1 KHz square wave output to the emitter

1‟s input.

b. Connect the fiber optic cable between the emitter‟s output and detector‟s

input.

c. Connect detector 1‟s output to the comparator 1‟s input.

d. Connect comparator 1‟s output to AC amplifier 1‟s input.

4. On the board switch emitter 1‟s driver to digital mode.

5. Switch ON the power.

6. Monitor both the inputs to comparator 1 (TP9 & 10). Slowly adjust the

comparator bias. Reset until DC level on the input (TP9) lies midway between

the high and low level of the signal on positive input (TP11).

7. Observe the input to emitter (TP 5) with output from AC amplifier 1 (TP 19)

and note that the two signals are same.

RESULT:

Page 65: Optical & Microwave Lab Manual

66

Connection Diagram:

Tabulation:

Sl.No. Distance of

Screen from

fiber(L cm)

Diameter of Spot

(W cm)

NA=SinѲmax

Ѳamax=Sin-1

(NA)

Function

Generator Emitter Circuit

To the

Numerical

aperture jig

Screen

Page 66: Optical & Microwave Lab Manual

67

NUMERICAL APERTURE DETERMINATION FOR

FIBERS

Aim:

To Measure the Numerical Aperture (NA) of an optical fiber

Equipments Required:

1. ST2501 Techbook with power supply cord

2. Optical Fibre cable

3. Numerical Aperture measurement Jig

Procedure:

1. Connect the Power supply cord to mains suppl.

2. Connect the Frequency Generator 1 KHz sine wave output to input of

emitter circuit. Adjust its amplitude at 5Vp-p.

3. Connect one end of fiber cable to the output socket of emitter circuit and

the other end to the numerical aperture measurement jig. Hold the white

screen facing the fiber such that its cut face is perpendicular to the axis of

the fiber.

4. Hold the white screen with 4 concentric circles (10, 15, 20 & 25 mm

diameter) vertically at a suitable distance to make the red spot from the

fiber coincide with 10 mm circle.

5. Record the distances of screen from the fiber end L and note the diameter

W of the spot.

6. Compute the numerical aperture from the formula given below.

7. Vary the distance between in screen and fiber optic cable and make it

coincide with one of the concentric circles. Note its distance.

8. Tabulate the various distances and diameter of the circles made on the

white screen and computer the numerical aperture from the formula given

above.

RESULT: